Application of calcium sulfate whiskers in radiation cooling materials

Abstract

This invention discloses the application of calcium sulfate whiskers in radiative cooling materials, relating to the field of radiative cooling technology. The invention discovers that calcium sulfate whiskers can be used as a radiative cooling material to achieve efficient radiative cooling. The preparation method of the calcium sulfate whiskers includes raw material pretreatment, segmented induced nucleation, gradient temperature ripening, and post-treatment, all performed under normal pressure conditions. Furthermore, this invention uses a specific combination of calcium sulfate whiskers and titanium dioxide to prepare a radiative cooling coating. This coating can construct an efficient light scattering network in the matrix, synergistically enhancing solar radiation reflection and atmospheric window infrared emission, achieving efficient radiative cooling while also exhibiting hydrophobic self-cleaning properties and high mechanical wear resistance, showing broad application prospects in the field of radiative cooling technology.

Description

Technical Field

[0001] This invention relates to the field of radiation refrigeration technology, and specifically to the application of calcium sulfate whiskers in radiation refrigeration materials. Background Technology

[0002] Radiative cooling is a zero-energy passive cooling technology. Its core lies in using the spectral properties of materials to efficiently reflect solar heat and radiate heat from outer space, thus achieving a cooling effect without consuming additional energy. Ideal passive radiative cooling materials must simultaneously meet two key optical conditions: first, high reflectivity in the solar spectrum (0.3-2.5μm) to shield solar heat; and second, high emissivity in the atmospheric infrared window (8-13μm) to enhance radiative heat dissipation.

[0003] In recent years, the preparation of radiation-cooling coatings by introducing crystalline fillers into polymer matrices has become a research hotspot. Chinese patent CN118994711A discloses a superhydrophobic porous radiation-cooling film using polydimethylsiloxane as a matrix and tetra-needle-shaped zinc oxide whiskers as fillers, demonstrating the feasibility of whisker-type fillers in radiation cooling. However, zinc oxide whiskers are costly and have a narrow infrared emission band. While Chinese patent CN121203454A lists calcium sulfate whiskers as one of the optional fillers, it does not reveal the influence of the diameter and aspect ratio of calcium sulfate whiskers on their radiation-cooling performance, only mentioning them generally among many fillers without optimizing their morphology and particle size, nor specifically studying their application effect in radiation cooling. Furthermore, while existing double-layer structure schemes (such as Chinese patent CN115851040A) can achieve efficient cooling, the construction process is complex, the interlayer interface affects long-term stability, and the coating's wear resistance is not adequately considered.

[0004] Therefore, developing a new integrated radiation cooling coating material that is simple to process, can synergistically achieve excellent cooling function, hydrophobic self-cleaning properties and high mechanical wear resistance is of great significance for broadening the application of radiation cooling technology. Summary of the Invention

[0005] Therefore, the present invention provides an application of calcium sulfate whiskers in radiation cooling materials to solve the problems in the prior art.

[0006] Radiation-cooling materials require complex crystal structures and appropriate chemical bond strength, possessing more infrared optical phonon modes within the atmospheric window spectral range to enhance their atmospheric window emissivity. Simultaneously, a wide electronic band gap is needed to reduce solar absorption. Furthermore, parameters such as the optical constants, volume fraction, and particle size of the radiation-cooling coating particles influence radiation characteristics such as attenuation, absorption, and scattering, further determining the spectral selectivity of the radiation-cooling coating. Moreover, research shows that cleverly designed micro / nano-structured surfaces can also effectively improve the spectral selectivity of radiation-cooling coatings. Therefore, designing radiation-cooling materials with optimal spectral selectivity is particularly important. This invention applies calcium sulfate whiskers to radiation-cooling coatings, utilizing their wide band gap and infrared reflectivity to effectively reduce the coating's absorptivity in solar radiation and increase its emissivity in the atmospheric window band. It also exhibits excellent thermal insulation, significantly limiting the influence of heat convection during heat transfer and effectively reducing environmental radiation energy input, demonstrating significant advantages in the design of radiation-cooling coatings.

[0007] This invention has discovered that calcium sulfate whiskers with specific diameters and aspect ratios, especially those prepared by a segmented induction process using titanium dioxide waste acid under normal pressure, have the characteristics of wide band gaps and excellent infrared reflectivity. They can effectively reduce the absorption rate of the coating in solar radiation and increase the emissivity in the atmospheric window band, while also having a good heat insulation effect, showing great advantages in the design of radiation cooling coatings.

[0008] To achieve the above objectives, the present invention provides the following technical solution: According to a first aspect of the present invention, a calcium sulfate whisker is used in a radiative cooling material. The calcium sulfate whisker has a diameter of 0.5-3 μm and an aspect ratio of 20-300; more preferably, the diameter of the calcium sulfate whisker is distributed in the range of 0.8-2.5 μm and the aspect ratio is 50-100. Applying this whisker to a radiative cooling coating can construct a highly efficient light scattering network in the matrix, synergistically enhancing solar light reflection and atmospheric window infrared emission, thus achieving highly efficient radiative cooling. Outdoor measurements show that the coating has a solar reflectivity ≥95%, an infrared emissivity of 8-13 μm atmospheric window ≥93%, and a cooling power ≥60 W / m². 2 Peak temperature drop ≥3.5℃, evening temperature drop ≥8.0℃.

[0009] Calcium sulfate whiskers with specific diameter distribution and aspect ratio can enhance infrared reflectivity. They can form more complex microstructures in materials, increase the reflection and scattering paths of infrared rays, improve the infrared reflectivity of radiative cooling materials, and thus enhance the cooling effect. On the other hand, they can better play a reinforcing role, improve the mechanical strength and stability of materials, and enable the durability and reliability of radiative cooling materials in practical applications.

[0010] This invention preferably uses high-quality calcium sulfate whiskers, requiring the whiskers to be highly pure, free of impurities, and have complete crystal forms to ensure the performance of the material.

[0011] Furthermore, the calcium sulfate whiskers are prepared by a method comprising the following steps: Step 1: Reduce and concentrate the waste acid from titanium dioxide to obtain a refined dilute sulfuric acid solution with a total iron ion concentration ≤3g / L. Step 2: Under normal pressure, calcium source solution is added dropwise to the refined dilute sulfuric acid solution in two stages. In the first stage, 20% to 40% of the total calcium source is added, and nucleation is induced at 65 to 75°C. After keeping warm, the remaining calcium source is added in the second stage, and crystal growth is carried out at 65 to 75°C. Step 3: Lower the system temperature to 50-60℃ and maintain the temperature for 2-4 hours. Step 4: After solid-liquid separation, washing, and drying, calcium sulfate whiskers are obtained.

[0012] As an example, the calcium sulfate whiskers are prepared by a method comprising the following steps: S1. Add a certain amount of iron powder to the titanium dioxide waste acid, stir and react at 55-60℃, and remove the Fe from the titanium dioxide waste acid. 3+ Reduced to Fe 2+ Then, filter while hot to obtain a clear filtrate; heat the filtrate to 85-95℃ and evaporate and concentrate it. When the density of the acid solution reaches 1.4-1.5g / mL, stop the concentration and filter while hot to obtain a purified acid solution. After cooling, adjust the sulfuric acid concentration to 1-2 mol / L with deionized water, and its total iron ion concentration is <3g / L. S2. Under normal pressure, take the purified acid solution obtained in step S1 and heat it to 65-75℃ (preferably 68-72℃). While stirring, add a CaCl2 solution with a mass concentration of 8%-12% in two stages at a uniform rate. The first stage involves adding 20%-40% of the total CaCl2 solution, and maintaining this temperature for 20-30 minutes to induce uniform crystal nuclei formation. Then, the second stage involves adding the remaining 60%-80% of the CaCl2 solution, and maintaining this temperature for another 25-35 minutes to achieve directional one-dimensional crystal growth. After the addition is complete, lower the system temperature to 50-60℃ and maintain this temperature for 2-4 hours to stabilize the crystal morphology. After cooling to room temperature, filter, wash, and dry to obtain calcium sulfate whiskers. In step S2, the total volume ratio of the CaCl2 solution added to the purified acid solution is (0.9-1.8):1.

[0013] The titanium dioxide waste acid is a hydrolyzed waste acid solution generated during the sulfuric acid process for preparing titanium dioxide. This invention involves a special pretreatment of the hydrolyzed waste acid solution generated in industrial production, by adding iron powder to remove Fe from the waste acid. 3+Reduced to Fe 2+ By utilizing the characteristic that the solubility of ferrous sulfate in sulfuric acid solution changes with temperature and concentration, waste acid is concentrated at high temperature to increase the concentration of sulfuric acid and ferrous sulfate. Due to the common ion effect and the decrease in solubility of ferrous sulfate monohydrate at higher temperatures, FeSO4·H2O crystallizes out when the solution density reaches 1.4-1.5 g / mL. The purified acid solution can be obtained by solid-liquid separation. This method can effectively remove iron-based impurities, and the total iron ion concentration in the purified acid solution is <3 g / L, which is beneficial for the subsequent preparation of calcium sulfate whiskers.

[0014] Furthermore, in step S1, the amount of iron powder used is based on the Fe content in the waste acid. 3+ The content is determined, and is usually 1.1-1.2 times the theoretical amount.

[0015] Furthermore, in step S1, the stirring reaction time after adding iron powder is 1-2 hours; the stirring speed is 100-150 rpm.

[0016] This invention employs a process combining segmented induced nucleation and gradient temperature ripening. First, a portion of the calcium source is added at a constant temperature to induce the formation of uniform crystal nuclei. After holding at this temperature, the remaining calcium source is added, achieving directional one-dimensional crystal growth at the same temperature. After the addition is complete, the temperature is lowered to 50-60℃ in a single step for ripening, stabilizing the crystal morphology. This process effectively separates the nucleation and growth stages, suppressing secondary nucleation, thereby obtaining calcium sulfate whiskers with high aspect ratio and uniform morphology.

[0017] The titanium dioxide waste acid used in this invention is the hydrolyzed waste acid solution produced during the sulfuric acid process for titanium dioxide preparation. Iron powder is added to remove Fe from the waste acid. 3+ Reduced to Fe 2+ Utilizing the characteristic that ferrous sulfate's solubility significantly decreases under high temperature and high acid conditions, FeSO4·H2O crystallizes out, and a purified acid solution is obtained through solid-liquid separation, with a total iron ion concentration <3 g / L. This pretreatment method effectively removes iron-based impurities, which is beneficial for the subsequent preparation of calcium sulfate whiskers. This invention employs an atmospheric pressure process combining segmented induced nucleation and gradient temperature ripening, effectively separating the nucleation and growth stages, suppressing secondary nucleation, thereby obtaining calcium sulfate whiskers with high aspect ratio and uniform morphology.

[0018] Furthermore, in step S2, the total volume ratio of the added CaCl2 solution to the purified acid solution is (1.2-1.6):1.

[0019] Further, in step S2, the addition rate of the CaCl2 solution is 1-2 L / min. In the method of the present invention, controlling the addition rate of the CaCl2 solution is beneficial for regulating the morphology of calcium sulfate whiskers. An excessively fast addition rate can lead to excessively high local supersaturation, generating a large number of fine crystal nuclei, easily forming short rod-shaped products, and decreasing the aspect ratio.

[0020] Further, in step S2, a crystal form control agent can be added to the purified acid solution before adding the CaCl2 solution; the crystal form control agent is preferably magnesium chloride. Further, the concentration of the crystal form control agent in the acid system is controlled at 0.03-0.04 mol / L.

[0021] Further, in step S2, the washing is performed using a hot calcium chloride solution at 55-60°C.

[0022] Further, in step S2, the drying is performed at 100-120°C until constant weight.

[0023] According to a second aspect of the present invention, a radiation cooling coating comprises calcium sulfate whiskers and a matrix material.

[0024] Furthermore, the matrix material is polydimethylsiloxane.

[0025] Furthermore, the coating also includes titanium dioxide. Titanium dioxide has a certain light absorption and emissivity in the mid-infrared band. When combined with calcium sulfate whiskers, it can enhance the material's radiative heat dissipation capacity in the infrared region and improve the radiative cooling effect. Simultaneously, both titanium dioxide and calcium sulfate whiskers possess good chemical stability and are not prone to chemical reactions with other substances. This allows the combined material to maintain stable performance under different environmental conditions, exhibiting good corrosion resistance and oxidation resistance. Titanium dioxide and calcium sulfate whiskers are relatively inexpensive and widely available, which helps reduce the overall cost of radiation cooling materials and enhance their competitiveness in the market.

[0026] Further, the average particle size of the titanium dioxide is 20-150 nm, preferably 80-120 nm. The average particle size of the titanium dioxide is analyzed according to standard GB / T 29022-2021. The titanium dioxide is selected from anatase titanium dioxide.

[0027] Furthermore, the mass ratio of polydimethylsiloxane, calcium sulfate whiskers, and titanium dioxide is 10:(1-2):(0.5-1). Calcium sulfate whiskers serve as the main structural support and the primary component for radiative cooling, while titanium dioxide acts as a reinforcing phase and an additive to assist in radiative cooling. The two are blended in a specific ratio to achieve efficient radiative cooling. If the titanium dioxide content is too low, its functions of enhancing mechanical properties and improving infrared radiation performance cannot be fully realized; if the titanium dioxide content is too high, it will affect the overall formability and uniformity of the material and may increase costs.

[0028] The method for preparing calcium sulfate whiskers in this invention uses titanium dioxide waste acid as raw material. Through a combined pretreatment of "iron powder reduction + concentration and refining," iron-based impurities are effectively removed, solving the problem of impurities interfering with crystal growth. The method employs an atmospheric pressure process combining segmented induced nucleation and gradient temperature ripening, eliminating the need for high-temperature, high-pressure hydrothermal equipment. This results in low safety risks, low equipment investment, and significantly reduced energy consumption. The process has a wide parameter window, good robustness, and is easy to scale up and achieve continuous production, exhibiting significant cost advantages and promising industrialization prospects.

[0029] This invention uses titanium dioxide waste acid, a waste product from the sulfuric acid process for titanium dioxide production, as the core raw material. Through resource-based conversion, it prepares high-performance radiation cooling functional fillers, which not only significantly reduces the raw material cost of calcium sulfate whiskers, but also provides a new path for the harmless treatment and recycling of waste acid in the titanium dioxide industry, in line with the strategic requirements of green chemical industry and sustainable development.

[0030] A method for preparing a radiation-cooling coating according to a third aspect of the present invention includes the following steps: S1. Pretreatment of calcium sulfate whiskers and titanium dioxide; S2. Add calcium sulfate whiskers and titanium dioxide to the matrix material, stir until a uniform paste is formed, then add solvent to dilute, and ultrasonically disperse to obtain a radiation cooling coating.

[0031] Further, the pretreatment method for calcium sulfate whiskers and titanium dioxide in step S1 includes: adding calcium sulfate whiskers or titanium dioxide to a 2%-5% (w / w) silane coupling agent ethanol solution, stirring and reacting at 50-70°C for 2-4 hours, then filtering and drying at 80-100°C for 4-6 hours.

[0032] Furthermore, in step S2, the solvent is selected from n-hexane.

[0033] Further, in step S2, the mass ratio of the solvent to the matrix material is (1-2):1.

[0034] Furthermore, in step S2, the ultrasonic dispersion time is 1-3 hours.

[0035] The application of the radiation-cooling coating provided in the fourth aspect of the present invention in the preparation of radiation-cooling coatings. The coating can be applied to surfaces of building materials, plastic and rubber products, textile materials, metal materials, etc., including but not limited to building exterior walls, industrial plants, equipment housings, open-air pipelines, agricultural greenhouses, automobile housings, spacecraft housings, electronic device housings, etc.

[0036] Specifically, the present invention provides a method for using the radiation cooling coating, comprising the following steps: applying the radiation cooling coating onto a substrate with a wet film thickness of 250-350 μm, and then curing it at 75-85°C for 1-3 hours to form a radiation cooling coating on the substrate surface.

[0037] The present invention further combines the calcium sulfate whiskers with titanium dioxide and selects polydimethylsiloxane as the matrix material. Through a specific ratio and pretreatment process, the resulting coating simultaneously achieves excellent hydrophobic self-cleaning properties (water contact angle > 130°) and high mechanical wear resistance (wear loss < 4.5mg), significantly extending the service life of the coating and broadening its application scenarios in harsh outdoor environments.

[0038] The present invention has the following advantages: This invention has discovered that calcium sulfate whiskers can be used as a radiation cooling material to achieve efficient radiation cooling. Furthermore, this invention employs a specific combination of calcium sulfate whiskers and titanium dioxide to prepare a radiation cooling coating. This coating can construct an efficient light scattering network within the matrix, synergistically enhancing solar radiation reflection and atmospheric window infrared emission, thus achieving efficient radiation cooling. Simultaneously, it maintains hydrophobic self-cleaning properties and high mechanical wear resistance, demonstrating broad application prospects in the field of radiation cooling technology. Attached Figure Description

[0039] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0040] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0041] Figure 1 This is a SEM image of a radiation-cooling coating provided in Embodiment 1 of the present invention. Detailed Implementation

[0042] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] Unless otherwise specified in the embodiments of this invention, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available products; different manufacturers and models of raw materials do not affect the implementation of the technical solution or the achievement of the technical effect of this invention.

[0044] Polydimethylsiloxane (PDMS): Dow Corning Sylgard 184. When using, mix component A (prepolymer) and component B (crosslinking agent) at a mass ratio of 10:1.

[0045] Titanium dioxide 1: Anatase titanium dioxide, with an average particle size of 100 nm, CAS: 13463-67-7, Aladdin reagent, catalog number T104946; Titanium dioxide 2: Anatase titanium dioxide, with an average particle size of 30 nm, CAS: 13463-67-7, Aladdin reagent, catalog number T299213; Calcium sulfate whiskers: can be prepared in-house or commercially available according to the method described in this invention, as detailed in the various embodiments.

[0046] Silane coupling agent: KH570, analytical grade.

[0047] Titanium dioxide waste acid: Hydrolysis waste acid liquid generated during the sulfuric acid process of titanium dioxide production at a certain titanium dioxide plant. The initial sulfuric acid concentration is approximately 2.8 mol / L, Fe²⁺ + The concentration is approximately 49.08 g / L, and the density is approximately 1.31 g / mL.

[0048] Other reagents: hexane, anhydrous ethanol, calcium chloride, iron powder, etc., are all commercially available analytical grade.

[0049] Preparation Example 1 (Calcium Sulfate Whiskers A) The method of this invention for preparing calcium sulfate whiskers comprises the following steps: S1. Take 1000 mL of titanium dioxide waste acid, add 8 g of elemental iron powder, control the temperature at 60℃, stir and react for 30 min, and then add Fe... 3+ Reduced to Fe 2+The insoluble residue was removed by filtration. The filtrate was heated to 90°C and concentrated to 500 mL (density approximately 1.43 g / mL) by negative pressure evaporation. It was filtered while hot, and after cooling, the sulfuric acid concentration was adjusted to 1.5 mol / L with deionized water to obtain a purified dilute sulfuric acid solution with a total iron ion concentration of 2.2 g / L.

[0050] S2. Add 500 mL of the above-mentioned purified dilute sulfuric acid solution to the reactor, heat to 70℃, and add 10% CaCl2 solution in two stages at a rate of 1.5 L / min while stirring, for a total addition of 500 mL. Add 20% (100 mL) in the first stage and keep warm for 20 min to induce nucleation; add the remaining 80% (400 mL) in the second stage and continue to keep warm for 30 min for crystal growth.

[0051] S3. After the addition is complete, lower the system temperature to 55℃ and let it mature at a constant temperature for 3 hours.

[0052] S4. Vacuum filtration, washing three times with 5% CaCl2 solution, rinsing twice with deionized water, and drying at 110℃ for 2 h yielded calcium sulfate whiskers A. SEM analysis showed diameters ranging from 0.8 to 2.2 μm, with an average aspect ratio of 85.

[0053] Preparation Example 2 (Calcium Sulfate Whiskers B) The only difference from Preparation Example 1 is that in S2, 40% (200 mL) of the total amount of CaCl2 was added in the first stage, and 60% (300 mL) was added in the second stage. Everything else was the same. Calcium sulfate whiskers B were obtained, with diameters ranging from 0.9 to 2.5 μm and an average aspect ratio of 72.

[0054] Preparation Example 3 (Calcium Sulfate Whiskers C) The only difference from Preparation Example 1 is that the reaction temperature in S2 was 65°C. Everything else was the same. Calcium sulfate whiskers C were obtained, with diameters of 1.0–2.5 μm and an average aspect ratio of 68.

[0055] Preparation Example 4 (Calcium Sulfate Whiskers D) The only difference from Preparation Example 1 is that magnesium chloride, a crystal form regulator, was added to S2 (the amount added was 0.03% of the mass of the purified dilute sulfuric acid solution). Calcium sulfate whiskers D were obtained, with a diameter of 0.8-2.0 μm and an average aspect ratio of 92.

[0056] Comparative preparation example 1 (calcium sulfate whiskers E) The only difference from Preparation Example 1 is that in S1, no iron powder is added, and no iron removal is performed; the waste titanium dioxide acid is simply diluted to the same sulfuric acid concentration before direct use. The resulting product is a mixture of short rods and granules with an average aspect ratio of <10, making it unsuitable as an effective filler for radiative cooling.

[0057] Comparison with commercially available calcium sulfate whiskers: Manufacturer: Guizhou Shengwei Chemical New Materials Research Institute Co., Ltd., Product Model: CSW-100; Aspect Ratio: 100.

[0058] Example 1 The calcium sulfate whiskers used were whiskers A from Preparation Example 1, and the titanium dioxide 1 had an average particle size of 100 nm. The proportions were: 100 parts PDMS, 15 parts calcium sulfate whiskers, and 10 parts titanium dioxide (by weight).

[0059] Preparation method: S1. Pretreatment: Calcium sulfate whiskers and titanium dioxide 1 were added to a 2% KH570 silane coupling agent ethanol solution, stirred at 70°C for 2 h, filtered, and dried at 100°C for 4 h.

[0060] S2. Coating preparation: According to the proportions in Table 1, add the pretreated calcium sulfate whiskers and titanium dioxide to PDMS, stir until a uniform paste is formed, add n-hexane of equal mass to PDMS for dilution, and ultrasonically disperse for 2 h to obtain the radiation cooling coating.

[0061] S3. Coating preparation: The coating is applied to a clean aluminum sheet with a wet film thickness of 300 μm. The film is left to stand at room temperature for 30 min to allow the solvent to evaporate, and then cured at 80℃ for 2 h to obtain the coating sample.

[0062] The radiation-cooled coating was observed using a scanning electron microscope (SEM), such as... Figure 1 The radiation cooling coating of the present invention, by employing a specific composite filler of calcium sulfate whiskers and titanium dioxide, can construct an efficient light scattering network in the matrix, synergistically enhancing the reflection of sunlight in the solar band and the infrared emission of the atmospheric window, thereby achieving efficient radiation cooling.

[0063] Example 2 The calcium sulfate whiskers used were whisker D (aspect ratio 92) from Preparation Example 4, and the rest were the same as in Example 1.

[0064] Example 3 The calcium sulfate whiskers used were whiskers B from Preparation Example 2, and the rest were the same as in Example 1.

[0065] Example 4 The calcium sulfate whiskers used were whiskers C from Preparation Example 3, and the rest were the same as in Example 1.

[0066] Example 5 The calcium sulfate whiskers used were whiskers A from Preparation Example 1, with the following proportions adjusted: 100 parts PDMS, 12 parts calcium sulfate whiskers, and 8 parts titanium dioxide. The rest was the same as in Example 1.

[0067] Example 6 The calcium sulfate whiskers used were whiskers A from Preparation Example 1, with the following composition: 100 parts PDMS, 18 parts calcium sulfate whiskers, and 12 parts titanium dioxide. The rest was the same as in Example 1.

[0068] Example 7 The calcium sulfate whiskers used were whiskers A from Preparation Example 1, and titanium dioxide 2 (average particle size 30 nm) was used instead of titanium dioxide 1. The rest was the same as in Example 1.

[0069] Comparative Example 1 The coating does not contain calcium sulfate whiskers, but only PDMS and titanium dioxide (100 parts PDMS and 10 parts titanium dioxide), and the rest is the same as in Example 1.

[0070] Comparative Example 2 The coating does not contain titanium dioxide, but only PDMS and calcium sulfate whiskers (100 parts PDMS and 15 parts calcium sulfate whiskers A), and the rest is the same as in Example 1.

[0071] Comparative Example 3 The whisker E (aspect ratio < 10, morphology of short rod-shaped particles) obtained by comparative preparation example 1 was used, and the rest was the same as in example 1.

[0072] Comparative Example 4 Commercially available calcium sulfate whiskers (5-10 μm in diameter, aspect ratio 15-25) were used, and the rest was the same as in Example 1.

[0073] Comparative Example 5 The formulation is as follows: 100 parts PDMS, 0.5 parts calcium sulfate whiskers A, 10 parts titanium dioxide, and the rest is the same as in Example 1.

[0074] Comparative Example 6 The formulation is as follows: 100 parts PDMS, 30 parts calcium sulfate whiskers A, 10 parts titanium dioxide, and the rest is the same as in Example 1.

[0075] Comparative Example 7 Calcium sulfate whiskers and titanium dioxide were mixed directly with PDMS without silane coupling agent pretreatment, and the rest was the same as in Example 1.

[0076] Comparative Example 8 Replace PDMS with commercially available acrylic resin, otherwise remain the same as in Example 1.

[0077] Comparative Example 9 The calcium sulfate whiskers used were commercially available calcium sulfate whiskers (diameter 1-3 μm, aspect ratio 50-80, product of a certain company), and the rest were the same as in Example 1.

[0078] Test Example 1 An outdoor testing platform was constructed. Tests were conducted on the cooling effect and cooling power of the radiation-cooling coating.

[0079] Solar reflectance: The reflectance in the 0.3-2.5 μm band was measured using a spectrophotometer (with integrating sphere) according to ASTM E903.

[0080] Infrared emissivity: The integrated emissivity in the 8-13 μm band was measured using a Fourier transform infrared spectrometer, according to ASTM E408.

[0081] Hydrophobicity: Using a contact angle meter (model: OCA20, DataPhysics), 5 μL of deionized water was dropped onto the coating surface, and the static water contact angle (WCA) was measured. The average value was taken at 5 different locations for each sample.

[0082] Abrasion resistance: A Taber abrasion tester (model: 5135, Taber Industries) was used, in accordance with standard ASTM D4060, using a CS-10 grinding wheel, with a load of 500g, and the sample was rubbed at a constant speed of 60 rpm for 500 revolutions. The mass of the sample was weighed before and after the test using a precision electronic balance (accuracy 0.1mg), and the mass loss (abrasion wear Δm) was calculated.

[0083] Radiation-cooling coating cooling and cooling power testing: An outdoor testing platform was constructed. The prepared coating sample (10cm×10cm) was placed side-by-side with a black anodized aluminum plate of the same size and thickness (absorptivity > 0.95) as a reference sample, with insulating foam tightly attached to the back to reduce heat conduction interference. A T-type thermocouple (accuracy ±0.1℃) was used to connect to a data acquisition instrument to monitor the temperature on the back of the sample and the air temperature in the shaded area in real time. Simultaneously, a solar radiometer was used to monitor solar irradiance. The test was conducted on a typical clear, cloudless day, and temperature data were recorded throughout the day. The net cooling power P was estimated using the steady-state heat balance equation, and the maximum temperature drop ΔT1 during peak solar irradiance and the temperature drop ΔT2 during the evening period were also recorded.

[0084] The results are shown in Table 1.

[0085] Table 1 Performance test results of coatings in each embodiment and comparative example

[0086] The coatings of Examples 1-7 of this invention all exhibit excellent comprehensive performance: solar reflectivity ≥94.0%, infrared emissivity ≥93.0%, water contact angle ≥147°, abrasion ≤2.3 mg, peak temperature drop ≥3.6℃, ​​and evening temperature drop ≥8.1℃. Among them, Example 2 (high aspect ratio whisker D) showed the best performance. Example 7, using 30nm TiO2, performed slightly worse than Example 1, which used 100nm TiO2, but was still better than most comparative examples, indicating that the particle size range of 20-150nm is effective. Comparative Example 1 (without whiskers) showed a significant decrease in reflectivity and emissivity; Comparative Example 2 (without TiO2) had lower optical performance than the Example; Comparative Example 3 (inferior whiskers) had the worst performance in all aspects; Comparative Example 4 (whisker diameter too large) showed a decrease in performance; Comparative Examples 5-6 (improper whisker ratio) showed insufficient optical performance when too little whisker was present, and decreased wear resistance when too much whisker was present; Comparative Example 7 (without silane pretreatment) showed a significant decrease in hydrophobicity; Comparative Example 8 (non-PDMS matrix) showed poor hydrophobicity and wear resistance; Comparative Example 9 (commercially available whiskers) showed better performance than Comparative Examples 3-4 but lower performance than the whiskers prepared in this invention, indicating that the whiskers prepared in this invention have better overall performance.

[0087] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. The application of calcium sulfate whiskers in radiation cooling materials, characterized in that, The calcium sulfate whiskers have a diameter of 0.5-3 μm and an aspect ratio of 20-300.

2. The application according to claim 1, characterized in that, The calcium sulfate whiskers are prepared by a method comprising the following steps: Step 1: Reduce and concentrate the waste acid from titanium dioxide to obtain a refined dilute sulfuric acid solution with a total iron ion concentration ≤3g / L. Step 2: Under normal pressure, calcium source solution is added dropwise to the refined dilute sulfuric acid solution in two stages. In the first stage, 20% to 40% of the total calcium source is added, and nucleation is induced at 65 to 75°C. After keeping warm, the remaining calcium source is added in the second stage, and crystal growth is carried out at 65 to 75°C. Step 3: Lower the system temperature to 50-60℃ and maintain the temperature for 2-4 hours. Step 4: After solid-liquid separation, washing, and drying, calcium sulfate whiskers are obtained.

3. A radiation-cooling coating, characterized in that, The coating comprises calcium sulfate whiskers and a matrix material.

4. The radiation cooling coating according to claim 3, characterized in that, The matrix material is polydimethylsiloxane.

5. The radiation cooling coating according to claim 4, characterized in that, The coating also includes titanium dioxide.

6. The radiation cooling coating according to claim 5, characterized in that, The average particle size of the titanium dioxide is 20-150 nm.

7. The radiative cooling coating according to claim 5, characterized in that, The mass ratio of polydimethylsiloxane, calcium sulfate whiskers and titanium dioxide is 10:(1-2):(0.5-1).

8. The method for preparing the radiation-cooling coating according to any one of claims 5-7, characterized in that, Includes the following steps: S1. Pretreatment of calcium sulfate whiskers and titanium dioxide; S2. Add calcium sulfate whiskers and titanium dioxide to the matrix material, stir until a uniform paste is formed, then add solvent to dilute, and ultrasonically disperse to obtain a radiation cooling coating.

9. The preparation method according to claim 8, characterized in that, The pretreatment method for calcium sulfate whiskers and titanium dioxide in step S1 includes the following steps: adding calcium sulfate whiskers or titanium dioxide to a 2%-5% silane coupling agent ethanol solution, stirring and reacting at 50-70°C for 2-4 hours, then filtering and drying at 80-100°C for 4-6 hours.

10. The application of the radiation-cooling coating according to any one of claims 3-7 in the preparation of radiation-cooling coatings.

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